21.3 Clinical Ultrasound Examinations: Indications, Patient Preparation, Scanning Protocols & Transducer Selection
Key Takeaways
- Transabdominal pelvic and obstetric first-trimester scanning requires a full urinary bladder as an acoustic window, whereas transvaginal scanning requires an empty bladder and provides higher resolution at shallower depth.
- Upper abdominal and biliary scanning requires fasting for about 6 to 8 hours so that the gallbladder is distended and bowel gas is reduced.
- Transducer frequency is chosen as a trade-off: high frequency of about 7 to 15 MHz gives superior resolution at shallow depth for thyroid, breast, testis and vascular access, while low frequency of about 2 to 5 MHz gives the penetration needed for abdominal, obstetric and cardiac work.
- The Focused Assessment with Sonography for Trauma examination interrogates four regions — right upper quadrant hepatorenal recess, left upper quadrant splenorenal recess, pelvis and subxiphoid pericardium — for free fluid.
- Ultrasound is the first-line modality in pregnancy, in paediatric and neonatal imaging, in the acute scrotum and in breast assessment under about 30 years of age, because it is non-ionising, real-time and portable.
21.3 Clinical Ultrasound Examinations: Indications, Patient Preparation, Scanning Protocols & Transducer Selection
Anatomical Presentation During Real Time Scanning is the largest sub-topic of Ultrasonography at 6 items, and its competencies are practical: explain the different ultrasound examinations as to rationale and indication, procedure and preparation, and appropriate devices and equipment, and perform the various examination and scanning procedures in medical diagnostic ultrasound. Sections 21.1 and 21.2 dealt with physics, instrumentation, artifacts and safety. This section is the clinical protocol.
1. Transducer Selection: The Governing Trade-off
Higher frequency gives better axial resolution but poorer penetration, because attenuation increases with frequency. Choose the highest frequency that still reaches the target depth.
| Transducer | Typical frequency | Footprint and beam | Principal uses |
|---|---|---|---|
| Curvilinear (convex) | 2-5 MHz | Wide, diverging sector at depth | Abdomen, obstetrics, general survey |
| Linear array | 7-15 MHz | Rectangular, high resolution, shallow | Thyroid, breast, testis, musculoskeletal, vascular access, superficial soft tissue |
| Phased array (sector) | 2-5 MHz | Small footprint, wide sector | Cardiac — fits between the ribs; also transcranial |
| Endocavitary (transvaginal / transrectal) | 5-9 MHz | High resolution at short range | Early pregnancy, gynaecological pelvis, prostate |
| Microconvex | 5-8 MHz | Small footprint | Neonatal head through the fontanelle, paediatric abdomen |
2. Preparation by Examination — The Highest-Yield Table
| Examination | Preparation | Why |
|---|---|---|
| Upper abdomen / hepatobiliary / gallbladder | Fast 6-8 hours (water permitted in some protocols) | A fasted gallbladder is distended and assessable; fasting reduces bowel gas |
| Renal / urinary tract | Moderately full bladder; no fasting strictly required | Distended bladder allows assessment of wall, jets and post-void residual |
| Transabdominal pelvis (gynaecological) | Full bladder — 4-6 glasses of water about an hour before, do not void | The bladder is an acoustic window that displaces bowel and lifts the uterus |
| Transvaginal pelvis | Empty bladder | A full bladder pushes the pelvic organs out of the near field |
| Obstetric first trimester | Full bladder if transabdominal; empty if transvaginal | Same acoustic-window logic |
| Obstetric second and third trimester | No preparation | Amniotic fluid is its own window |
| Thyroid / neck | None; neck extended with a pillow under the shoulders | Access to the lower poles |
| Breast | None; no deodorant or lotion is required for ultrasound | Direct contact scanning |
| Scrotal / testicular | None; scrotum supported on a towel between the thighs, penis retracted and covered | Stabilises the target |
| Musculoskeletal | None | Dynamic scanning during movement is often diagnostic |
| Vascular / carotid / venous duplex | None; avoid heavy meals before mesenteric studies | Bowel gas obscures deep vessels |
| Echocardiography | None | — |
| Neonatal cranial | None; scan through the anterior fontanelle | The fontanelle is the acoustic window and closes by 12-18 months |
| Interventional guidance (biopsy, drainage, cannulation) | Coagulation status checked; sterile probe cover and sterile gel | Aseptic technique is mandatory |
The single most examined pairing: transabdominal pelvic scanning requires a full bladder; transvaginal scanning requires an empty bladder.
3. Protocols by Region
Hepatobiliary and upper abdomen
- Liver: longitudinal and transverse sweeps through both lobes; subcostal, intercostal and epigastric approaches; document the right and left lobes, portal and hepatic veins, and the hepatic echotexture. Normal liver is slightly more echogenic than the renal cortex.
- Gallbladder: long axis and transverse, supine and left lateral decubitus to demonstrate mobility of a calculus. A stone is echogenic, casts a clean posterior acoustic shadow, and moves with gravity; a polyp is echogenic, non-shadowing and non-mobile. Wall thickness above about 3 mm is abnormal. The sonographic Murphy sign — maximal tenderness with the probe directly over the gallbladder — supports acute cholecystitis.
- Biliary tree: common bile duct measured at the porta hepatis; normal diameter is about 6 mm, allowing roughly 1 mm per decade over 60 years and a larger calibre post-cholecystectomy.
- Pancreas: transverse epigastric with the splenic vein as the posterior landmark; water loading may improve the window.
- Spleen: left intercostal coronal; normal length up to about 12 cm.
- Aorta: longitudinal and transverse from the diaphragm to the bifurcation; measure outer wall to outer wall; the upper limit of normal is about 3 cm.
Renal and urinary
Longitudinal and transverse of each kidney with the patient supine and in the decubitus positions. Document length (normal about 9-12 cm), cortical thickness, corticomedullary differentiation and the collecting system. Normal renal cortex is less echogenic than liver and spleen. Bladder volume and post-void residual are calculated from three orthogonal measurements. Hydronephrosis is graded by the degree of pelvicalyceal dilatation and cortical thinning.
Obstetric
- First trimester: confirm intrauterine location, number, cardiac activity, and date by crown-rump length, which is the most accurate dating measurement. The gestational sac is visible transvaginally at about 5 weeks and the yolk sac shortly after.
- Second and third trimester: biometry — biparietal diameter, head circumference, abdominal circumference, femur length — plus fetal anatomy survey, placental location and grade, amniotic fluid assessment, and umbilical artery Doppler where indicated.
- Always confirm fetal cardiac activity and document it; and always check for placenta praevia in relation to the internal os.
Gynaecological
Uterine size, position, endometrial thickness and echotexture; both ovaries with follicle assessment; free fluid in the pouch of Douglas. Endometrial thickness varies with the cycle, so record the last menstrual period.
Thyroid and neck
Both lobes and the isthmus in transverse and longitudinal planes; document nodule size in three dimensions, composition, echogenicity, margins, and the presence of microcalcifications; survey the cervical nodal levels.
Scrotal
Both testes in transverse and longitudinal planes, plus a transverse image including both testes on one frame for direct comparison of size and echogenicity. Colour and spectral Doppler are mandatory — absent intratesticular flow in an acutely painful testis is torsion and is a surgical emergency. Epididymal head, body and tail; document any hydrocoele or varicocoele, the latter with a Valsalva manoeuvre.
Focused Assessment with Sonography for Trauma (FAST)
Four windows, looking for free fluid:
- Right upper quadrant — hepatorenal recess (Morison pouch), the most dependent space in the supine patient and the commonest site of a positive finding.
- Left upper quadrant — splenorenal recess and left subphrenic space.
- Pelvis — retrovesical or rectouterine pouch.
- Subxiphoid — pericardium, for tamponade.
The extended FAST (eFAST) adds bilateral anterior chest windows for pneumothorax, identified by absence of lung sliding and a stratosphere or barcode sign on M-mode.
Vascular
Carotid duplex documents intima-media thickness, plaque and peak systolic velocities in the common, internal and external carotid arteries and the vertebral artery. Lower-limb venous duplex tests compressibility at defined levels — a non-compressible vein is the primary sign of deep vein thrombosis — with augmentation and colour Doppler as adjuncts.
4. Optimising the Image While Scanning
| Control | Effect |
|---|---|
| Depth | Set so the region of interest fills the image; excess depth wastes resolution and lowers frame rate |
| Focal zone | Place at or just below the region of interest; multiple zones improve lateral resolution but lower frame rate |
| Overall gain | Uniform brightness; too high creates noise, too low loses low-level echoes |
| Time gain compensation (TGC) | Compensates for attenuation with depth so a uniform organ appears uniformly bright top to bottom |
| Frequency | Highest that still penetrates |
| Harmonic imaging | Reduces near-field reverberation and clutter, especially in larger patients |
| Compound imaging | Reduces speckle and improves margin definition |
| Doppler angle | Keep at or below about 60 degrees to the flow direction; velocities become unreliable above that |
| Pulse repetition frequency / scale | Raise for high velocities to prevent aliasing; lower for slow flow |
5. Patient Care During the Examination
Explain the procedure and obtain consent; warm the gel where possible; drape for dignity and expose only the region being scanned; offer a chaperone for transvaginal, transrectal, breast and scrotal examinations and document the offer; scan with the patient positioned comfortably and safely; clean the transducer between patients according to the disinfection level required — high-level disinfection and a single-use cover for endocavitary probes; and remove gel and assist the patient afterwards.
Scope note. In Philippine practice, sonographic interpretation is a medical act. The radiologic technologist performs the examination, produces and documents the required images and measurements, and reports technical observations — but does not issue a diagnosis to the patient.
A 24-year-old woman is booked for a transabdominal pelvic ultrasound followed, if needed, by a transvaginal study. What bladder preparation should be given?
Which transducer is most appropriate for adult transthoracic echocardiography, and why?
During a FAST examination on a hypotensive trauma patient, which window is most likely to reveal free intraperitoneal fluid first in a supine patient?
A patient is referred for gallbladder ultrasound after a large breakfast. Why should the examination be rescheduled?